Mira Variables from the OGLE II Galactic Disc Photometric Database in Centaurus and Norma

J. Greaves
Northants, England, UK

Draft in Preparation;   accepted:   6.11.2008
(E-mail for contact: bydra@safe-mail.net)

Comments:


24. First noted as a variable star in Derut et al. (2002).

27. First noted as a variable star in Derut et al. (2002).

29. First noted as a variable star in Derut et al. (2002).

30. First noted as a variable star in Derut et al. (2002).

31. First noted as a variable star in Derut et al. (2002).

Remarks:
The OGLE On-line Photometric Databases (Szymanski 2005) carry epoch photometry from the OGLE II surveys (Udalski et al. 1997) released to the public domain, and including a survey pointed at some fields in the Galactic Disc for which apparently no variability study has been published despite being available for several years.

Of lesser duration than the Bulge and Magellanic Cloud surveys, this dataset nevertheless can be seen to contain nearly one million stars fulfilling the criterion of having about one hundred plus observations each and is spread over an approximately one thousand day interval. This is sufficient for the assessment of period and variability type for many types of variable star, especially periodic ones, and even long period variables such as Mira stars.

An examination of the database returned 78 certain Mira variables, 32 from the Centaurus and Norma fields are presented here. A further 46 can be found in the Scorpius fields. The Carina fields appeared to have no readily apparent certain Mira variables that could be readily distinguished from semiregular variables. This compares well with a survey for Miras (Groenewegen and Blommaert 2005) conducted upon the admittedly slightly denser OGLE II Bulge fields of roughly thirty million stars, which reported nearly three thousand Mira variables. That is, the fraction of Miras is an order of four less in these two cases.

This list of 32 Mira variables gives details of the stars. As the OGLE II experiments use a passband that is very near to Cousins I, the amplitude of variability for these Miras is somewhat suppressed relative to situation with Johnson V, which is normally the case (see for example Lockwood and Wing (1971) for I band lightcurves of well known classical Miras), but the morphology of the lightcurves make the class evident enough. Another similarity to the Lockwood and Wing paper is the fact that despite a passband being used that is away from the opaque Titanium Oxide absorption lines, the lightcurves do not necessarily repeat cleanly from one cycle to the next, whether in peak magnitude or in ascending branch hump location or in cycle to cycle interval.

The seasonal nature of the survey, the short duration relative to the periods involved, and the magnitude limits, all combine to not allow coverage of more than three to four cycles at most for these objects, with maxima and minima being missing in some instances. Nevertheless period analysis of the data allows some good and very good periods to be determined, as well as approximate ones, and if minima are used in addition to the traditional maxima, epoch information can also be gleaned.

None of the stars are noted in the GCVS, NSV (http://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/html/ ) nor ASAS variable star listings (http://www.astrouw.edu.pl/asas/?page=download ), the resources most likely to cover these regions, which is to be somewhat expected as Mira variables will on the whole have V-Ic colours of 2, 3 or higher, thus often being much fainter in the more traditionally used Johnson V and visual ranges used in traditional variability surveys. Five stars (Nor_SC4 208572, Nor_SC7 59850, Nor_SC7 128069, Nor_SC8 46758, and Nor_SC8 156431) were found variable in the EROS II project (Derut et al. 2002) but not studied in any detail.

References:
Derue, F., Marquette, J.-B., Lupone, S., et al., 2002, Astron. Astrophys., 389, 149
Groenewegen, M.A.T., Blommaert, J.A.D.L., 2005, Astron. Astrophys., 443, 143
Lockwood, G.W., Wing, R.F., 1971, Astrophys. J., 169, 63
Szymanski, M., 2005, Acta Astronomica, 55, 43
Udalski, A., Kubiak, M., Szymanski, M., 1997, Acta Astronomica, 47, 319